The present invention is related to the field of exhaust gas recirculation control systems for internal combustion engines, and more particularly is related to the field of exhaust gas recirculation control systems for diesel engines for automotive vehicles and the like.
Diesel engines generally operate on an over lean mode, i.e. more intake air is sucked in via the intake system of the engine than is required for combustion of the fuel supplied to the combustion chambers thereof by the fuel injectors which are mounted in said combustion chambers. The amount of this sucked in air which is in excess of the amount of air required to burn the fuel is conventionally called the "excess air". It has been conceived, and practiced, to replace a part of this excess air sucked into the intake system of the diesel engine by exhaust gas recycled into the intake system of the engine from its exhaust system, in order to reduce the level of nitrogen oxide pollution in the exhaust gases. This so called exhaust gas recirculation is effective for keeping the levels of pollutants down to reasonable values. In other words, the basic point of exhaust gas recirculation in diesel engines is to replace at least part of the excess air by recycled exhaust gases. For maximum effectiveness in combating emission of harmful pollutants, it is preferable that a certain part of the excess air should be replaced by exhaust gases, so that the exhaust gases of the diesel internal combustion engine may be well purified of nitrogen oxides, and also the engine is inhibited from producing excessive exhaust smoke, and is maintained in a good engine operability condition.
Generally, the excess air ratio of a diesel engine is related with the engine load, i.e., to the amount of diesel fuel being injected per single injection into the combustion chambers of the engine. That is to say, the lower is the engine load, the greater is the excess air ratio; and, conversely, as engine load increases, the excess air ratio decreases. Therefore, in order to conduct the exhaust gas recirculation at a flow rate corresponding to the excess air amount, it is desirable to reduce the exhaust gas recirculation ratio according to increasing engine load. This exhaust gas recirculation ratio is defined as the value produced by dividing the exhaust gas recirculation flow rate by the sum of the exhaust gas recirculation flow rate and the air intake flow rate; i.e., is defined as the ratio of the exhaust gas recirculation flow rate to the total flow rate of gas intake by the combustion chambers of the diesel internal combustion engine.
As mentioned above, in a diesel engine, since the load thereon is effectively controlled by the amount of fuel injected into the combustion chambers thereof through the fuel injectors, and there exists substantially no intake manifold negative pressure within the intake system thereof (any negative pressure present in said intake manifold in any case not being directly correlated with the load on the diesel internal combustion engine), the type of exhaust gas recirculation control valve for gasoline internal combustion engines which is driven by a diaphragm device which is actuated by the intake manifold negative pressure cannot be used for diesel internal combustion engines. Therefore, some other way of relating the opening amount of an exhaust gas recirculation control valve, which controls the effective cross sectional opening area of an exhaust gas recirculation passage which returns a part of the exhaust gases from the exhaust system of the diesel engine to the intake system thereof, to the engine load, is required to be found.
It has previously been conceived, in diesel engines, mechanically to connect a movable member, such as a lever mounted on the valve stem of an exhaust gas recirculation control valve, to a movable part of an accelerator pedal linkage of the vehicle incorporating the diesel engine, or alternatively to a control lever for a fuel injection pump which provides fuel to the fuel injectors of the engine, either of which parts moves according to the amount of fuel per injection being provided to the diesel engine, i.e. according to the load on said engine. This connection has been performed mechanically using wires and links, etc., and thus indirectly drives the exhaust gas recirculation control valve according to the displacement of the above mentioned movable element, i.e. according to the load on the diesel engine. However, there are difficulties with regard to this proposed structure.
First, since the exhaust gas recirculation control valve and the aforesaid movable member are merely directly connected, in the above proposed structure, it is difficult for the performance of variation of the exhaust gas recirculation ratio with respect to the load to be freely chosen, and in fact a wide freedom for choice in said variation performance is very desirable, from the viewpoint of obtaining best operativity and exhaust emission quality from the diesel engine.
Secondly, with such a mechanical structure as proposed above, if it is desired to superpose onto the elementary relationship between the exhaust gas recirculation ratio and the load on the diesel engine a correcting factor related to, for example, the engine rotational speed, or the engine temperature, or the like, then it is not practical to incorporate such a correcting factor in the mechanical linkage, and it becomes necessary, for example, to provide a shut off valve or a flow amount control valve within the exhaust gas recirculation conduit, separately from the exhaust gas recirculation control valve. This is very troublesome, and causes additional cost in producing the system, and also causes unreliability during its operation.
Third, since the pressing of the foot of the driver of the vehicle upon the accelerator pedal thereof is required, in the above proposed construction, not only to actuate the movable member and to control a fuel metering element of the fuel injection pump, but is also required to move the valve element of the exhaust gas recirculation control valve which is mechanically connected thereto, the load upon the accelerator pedal of the vehicle becomes unacceptably heavy, and the driving feeling for the vehicle is deteriorated.
Because of these various problems with regard to exhaust gas recirculation control systems in which the above described movable member is physically mechanically connected to the valve element of the exhaust gas recirculation control valve, up until now such a system has not been usable in practice, and some alternative has been required to be found.